The present invention relates generally to a hydraulic control system for a hydraulically operated clutch of a power transmission of a motor vehicle and particularly to a hydraulic control system of this type which is improved to comprise means for, when a parameter representative of a function of the output torque of an engine of the vehicle in accordance with which parameter regulator means regulates a control pressure for the clutch abruptly varies and fails to represent the function during deceleration, delaying the transmission of variation in the parameter to the regulator means to prevent the control pressure from being regulated to vary at the same rate as that of variation in the parameter so that the clutch capacity is prevented from being temporarily insufficient with respect to the engine output torque to prevent the generation of an abnormal noise and a shock.
As is well known in the art, a hydraulic control system for a power transmission of a motor vehicle is provided with regulator means for regulating a control or working pressure for a hydraulically operated clutch in accordance with a parameter such as an intake passageway vacuum representative of a function of the output torque of an engine of the vehicle. This is to control the capacity of the clutch to a necessary minimum value in accordance with the output torque of the engine and to reduce a shock generated in the transmission during shifting of a gear ratio. In this instance, as the margin of the clutch capacity is reduced, a shock produced during change of a gear ratio is reduced.
However, a power transmission having a conventional hydraulic control system has been encountered by an inconvenience that an abnormal noise and/or a shock is produced when the accelerator pedal is returned toward its zero output position to decelerate the vehicle just after its sudden start or during its ascending travel for allowing a following vehicle to outstrip the preceding vehicle or for avoiding a collision with a retrograding vehicle. Such a noise and shock occur because, since although the parameter such as the intake passageway vacuum quickly complies with or responds to abrupt movement of the accelerator pedal toward its zero output position, the output torque or load of the engine slowly responds to the abrupt movement of the accelerator pedal, the capacity of the clutch is instantaneously or temporarily insufficient with respect to the engine output torque to create a slippage in the clutch.
As a measure for preventing the slippage being produced in the clutch, there is a notion of increasing the margin of the clutch capacity, for example, by increasing a preset value of the working pressure or the number of the clutch plates. However, these measures result in the occurrence of a shock during change of a gear ratio and an increase in the production cost.